1. Technical Field
The present invention relates to a power generation system using molten carbonate type fuel cells.
2. Background Art
A fuel cell directly transforms chemical energy of fuel into electrical energy and there are proposed a lot of power generation systems using fuel cells. A conventional molten carbonate type fuel cell system will be explained with reference to FIG. 7 of the accompanying drawings. A fuel cell 1 includes a stack of fuel cell elements with separators being interposed between two adjacent fuel cell elements. The fuel cell element is generally composed of an anode, a cathode, an electrolyte sandwiched by the anode and the cathode. Anode gas passages and cathod gas passages are formed in the separators. Air A is compressed by a compressor 4, then cooled by a cooling device 5, compressed by another compressor 6, preheated by an air preheater 7 and led into a cathode chamber 2 of the fuel cell 1 through a line 8. This is an oxidizing gas introduction to the cathode chamber 2. A part of the air A flowing through the line 8 is branched to a reformer 10 by a line 9. Gases discharged from the cathode chamber 2 are led into a turbine 12 through a line 11 and then introduced into the air preheater 7 before expelled. Part of the gases discharged from the cathode chamber 2 is fed back to an cathode chamber entrance side by a recycle line 25 and a recycle blower 26.
Gases discharged from the anode chamber 3 of the fuel cell 1 contain moisture. Thus, the moisture is usually separated from the gases before used againg in the cycle. For this purpose, the anode gas discharged from the anode chamber 3 of the fuel cell 1 is forced to flow through a heat exchanger 13, preheaters 14 and 15, an evaporator 16, a condenser 17 and a gas-liquid separator 18. The anode gas is cooled in the heat exchanger 13 and is subjected to a heat exchange with natural gas NG in the preheaters 14 and 15. The anode gas is condensed in the condeser 17 and the moisture and gases are separated from each other by the gas-liquid separator 18. The gases separated are introduced into the reformer 10 by the blower 19 through a line 20 extending to the heat exchanger 13. Water (H.sub.2 O) is pressurized by a pump 21 and sent to a water heater 22. The water is heated to steam by the heater 22 and led to the reformer 10 via a line 23 and the evaporator 16. The water is mixed with the natural gas NG in the reformer 10. Fuel produced in the reformer 10 is introduced to the anode chamber 3 of the fuel cell 1 through a line 24 whereas gases discharged from the reformer 10, which gases containing carbone dioxide, are introduced to the cathode chamber 2 of the fuel cell 1 with gases flowing in the line 8.
Heat is produced in the fuel cell 1 as the fuel cell power generation system is operated. Thus, in order to cool the fuel cell 1, a ratio of gases introduced to the anode chamber 3 and the cathode chamber 2 is adjusted to about 1:10. In other words, an amount of gases to be introduced to the cathode chamber 2 becomes larger than that of gases introduced to the anode chamber 3 so that the cathode is cooled by a large amount of air. A temperature difference between the entrance and the exit of the cathode chamber 2 is generally 100.degree. to 150.degree. C. This is a small difference. Therefore, a large amount of air is necessary for cooling, namely air of five to ten times necessary for reaction is required.
FIG. 8 shows a fundamental construction where air is used for cooling the cathode. The line 8 which supplies the cathode gas to the cathode chamber 2 is provided with the air preheater 7 and the cathode exhaust gas is fed to the air preheater 7 by the line 11.
Following passages deals with a case where the entrance temperature of the cathode chamber 2 is 600.degree. C., the exit temperature of the cathode chamber 2 is 700.degree. C. and the difference therebetween is 100.degree. C.
Meanwhile, in a case where a large amount of air is required for cooling the cathode, the system employing the air preheater 7 like the one illustrated in FIG. 8 has following disadvantages:
(1) The air preheater 7 has to be large in size. Accordingly, a profitability of a power generation plant is deteriorated in terms of cost and volume and the piping and the blower become large;
(2) High temperature exhaust gases of the fuel cell cannot be used. Specifically, a high temperature portion of the gases discharged from the cathode chamber 2 is used for preheating the gases to be fed to the cathode chamber 2 in the air preheater 7. Therefore, it is not possible to effectively use the hot exhaust gas discharged from the cathode chamber 2; and
(3) A partial pressure of CO.sub.2 drops as an amount of air increases. Therefore, a large voltage (potential difference) cannot be expected.
On the other hand, a construction of FIG. 9 is also proposed. In FIG. 9, with comparison to FIG. 8, the air preheater 7 is not used. Instead, a part of the cathode gas discharged from the cathode chamber 2 is recycled to the entrance of the cathode chamber 2 by the recirculation line 25 and the recirculation blower 26. The high temperature gases discharged from the cathode chamber 2 is mixed with air introduced to the cathode chamber 2, thereby adjusting the temperature. In this case, the energy consumption of the recycle blower 26 becomes large a problem of safety arises as an amount of cooling air increases.
As described above using a large amount of air in cooling the cathode raises a lot of problems.